1. Field of the Invention
The present invention pertains to high performance integrated analog circuits. More particularly, the present invention is directed to transconductor circuits having single-ended inputs.
2. Discussion of Related Art
Many personal communications devices, such as pagers as well as cellular voice and data terminal devices, are already available in pocket-size models. The construction of such devices is rapidly approaching "wallet-size" dimensions, which requires micro-miniaturization of both the circuits and the power cells used by such devices. Such micro-miniaturized circuits include high-performance analog audio and radio frequency (RF) circuit elements which must operate on the low power levels available from corresponding miniature power cells, and must operate within an operating range which may be only two volts relative to ground, at most. In particular, the audio circuits must provide linear response to voice signals without losing the dynamic range required for reproducing expressive speech.
Linear integrated field-effect-transistor (FET)-based transconductor circuits for small-signal differential inputs are known, such as the circuits disclosed in U.S. Pat. No. 5,568,091 to Ramalho et al. and U.S. Pat. No. 5,751,177 to Tanoi. Some known circuits provide substantially rail-to-rail compliance with the input signal at their output terminals, such as described in the Ramalho U.S. Pat. No. 5,568,091 patent. On the other hand, single-ended transconductor circuits, such as circuits similar to the generic transconductor circuit 5 shown in FIG. 1, are not as easily implemented. In particular, although some differential transconductor circuits provide for low-power operation by eliminating transconductance elements from the tails of a differential amplifier pair used by the transconductor circuits, single-ended circuits require a large value for the resistance R.sub.L in order to minimize power consumption according to : EQU I.sub.OUT =(V.sub.IN -V.sub.t)/R.sub.L.
In an example implementation, the load resistor 14, such as a polysilicon or N-tub resistor, has a load resistance R.sub.L of about 1 M.OMEGA. connected to the input amplifier 12 in FIG. 1. Implementing a load resistor 14 with such a high resistance value typically requires a large amount of "real estate" on the surface of a integrated circuit implementing the transconductor circuit 5. Such an integrated circuit would be awkwardly large and, more important, too costly to be practical for implementing low-power transconductor circuits 5 in pocket and wallet-size models of the personal communications devices. Furthermore, although metal oxide semiconductor FET (MOSFET)-based integrated circuits are well suited to miniature, low-power implementations of circuits for personal communications devices, such MOSFET integrated circuits have threshold offsets that severely reduce their dynamic operating range, and such circuits operate with non-linear characteristics for much of the operating range between that threshold offset and saturation.